Interaction of the P450 enzymes with their redox partners and the mechanism of electron transfer are the most important and intriguing problems in the area of P450 research. The crystal structures of four bacterial P450s, the catalytical domain of microsomal P450 reductase, and the NMR structure of putidaredoxin are known. However, knowledge of these structures does not directly answer where and how the proteins interact. Since interaction of other electron transfer proteins in crystallographic electron transfer complexes was shown to be specific and relevant to that occurring in a solution, we propose that crystallization of P450-redox partner complexes will enable the identification of interaction sites, conformation changes occurring upon binding, and the electron pathway from one co-factor to another. The P450cam electron transfer system and the heme/FMN-binding domain of flavocytochrome P450BM-3, a functional and structural analog to microsomal P450s and reductase, are the best candidates for crystallization trials of Class I and Class II monooxygenase systems, respectively. Having crystallized both the covalently linked heme/FMN-binding domain and the non-covalent complex between the separate heme-and FMN-binding domains of P450BM-3, we are in an excellent position to solve these structures, which will, undoubtedly, lead to our better understanding of the mechanism and function of microsomal P450-dependent monooxygenase systems.